To meet today's demand for high-speed cost-effective communications, optical transmission systems having increased data capacity are highly desirable. One approach used in modern high-capacity transmission systems to increase the aggregate data-rate of transmission is to use a technique called dense wavelength division multiplexing (DWDM). In DWDM, an optical transmission link is divided into a plurality of channels with each channel having its own center frequency. Data transmitted on a particular channel is then effected by modulating the optical carrier at the center frequency of that channel. At the receiver, a band-pass filter tuned to the center frequency of the channel is used for detecting and demodulating the transmitted signal. By combining a plurality of channels in this manner, the aggregate data capacity of the optical link is increased.
In DWDM transmissions systems, a separation between adjacent channels sufficient to reduce cross-channel interference to acceptable levels is required. Channel separations in the range of 100 GHz are commonly used to achieve sufficient separation. The data capacity of DWDM systems can be further increased by reducing channel separation to e.g. 50 or 25 GHz. The level to which channel separation can be reduced while still maintaining acceptable system performance is dependent in part on the stability of the laser source that generates the optical carrier. For example, if the optical carrier frequency drifts excessively as a result of temperature changes or aging of the laser source, cross-channel interference will increase especially if a smaller channel separation is used. To reduce the occurrence of cross-channel interference caused by frequency drifts, a control system for stabilizing the frequency/wavelength of the optical carrier generated by the laser crystal may be used.
Prior art systems exist in which a feedback loop is employed for stabilizing the output of a laser source. In these prior art systems, the optical beam generated by the laser crystal is passed through a filter and received by a detector which converts the filtered optical signal to a DC electrical signal whose amplitude is a function of the wavelength of the optical beam. The electrical signal is then processed and fed back to the laser source in a manner that controls the laser source to produce an optical beam having the desired wavelength. The laser source can be controlled using any number of the operating parameters of the laser source including temperature, voltage and current. In this way, a control system is provided to stabilize the output frequency of a laser source that may otherwise drift because of aging of the laser source and/or temperature changes.
A drawback of the prior control systems is that the signals used to provide feedback control are DC signals or, at the most, very slowly changing. As a result, DC amplifiers are required to process the control signals which may introduce errors caused by noise, drift, leakage currents and offset voltages that are typically associated with processing DC signals. These errors may interfere with system's ability to stabilize the output of the laser source. Therefore, it is desirable to provide a control system for stabilizing the frequency output of a laser source which reduces the errors associated with processing DC signals.